The present invention relates to a method and apparatus for detecting a wavelength of a laser beam, and particularly, to apparatus of the type which uses etalon.
With recent trend of high density of LSI, attention has been paid to a lithography in which an excimer laser is formed into narrow band to form a wiring pattern. However, the excimer laser has a problem in that the wavelength stability is poor in terms of construction thereof. Therefore, it is necessary to stabilize the wavelength. As a general apparatus for stabilizing a wavelength, there is a method for measuring the wavelength of oscillation light, and applying feedback control to a laser oscillator in response to the measured result.
Since the wavelength of laser is delicately varied due to external turbulances such as using environment, mechanical vibrations and the like, it is a key how the wavelength is measured with high precision for stabilization of the oscillation wavelength. There are various methods for measuring the laser wavelength. A method which is relatively simple and can obtain a precision is one which uses an etalon. In this method, a laser beam is incident upon etalon, and a half portion of an interference stripe 20 depicted by the permeated light is irradiated on one dimensional photodetector array 21 as shown in FIG. 5. The one dimensional photodetector array comprises a number of light receiving elements such as CCDs arranged and outputs light and shade of the interference stripe 20 as the intensity of signal. Various processes are applied on the basis of addresses applied to these light receiving elements to obtain the wavelength.
The etalon varies in refractive index according to variations in temperature and humidity. Therefore, there is a method for correcting an error using a reference laser oscillation light having a reference wavelength. This method comprises introducing a reference laser oscillation light and a measured-laser oscillation light into an etalon, and measuring a positional difference between a concentric interference stripe caused by the reference laser oscillation light and a concentric interference stripe caused by the measured laser oscillation light to thereby measure the wavelength of the measured-laser oscillation light. In this case, the reference laser oscillation light and the measured-laser oscillation light are simultaneously or individually incident upon and measured.
However, in the aforementioned conventional methods, the position for measuring the interference stripe becomes deviated unless the relative positional relationship between the interference stripe made by the etalon and the one dimensional photodetector array is always constant. However, it is not easy to always keep such a relative positional relationship constant. Actually, there is a problem in that an error caused by the deviation of position is large.
For example, in the case where not only a simple error in adjustment of position but also a thermal expansion or mechanical vibration is applied to the apparatus, a positional error between the interference stripe and the one dimensional photodetector array occurs so that the interference stripe is irradiated on the portion different from a normal position, as a consequence of which a deviation occurs in the addresses of the light receiving elements.